1. Field of the Invention
The present invention relates to a surface acoustic wave branching filter including first and second surface acoustic filters having different passbands and more particularly, the present invention relates to a surface acoustic wave branching filter of a ladder-type circuit structure having a plurality of series-arm resonators and a plurality of parallel-arm resonators.
2. Description of the Related Art
Up to now, various surface acoustic wave branching filters using a surface acoustic wave filter of a ladder-type circuit structure having series-arm resonators and parallel-arm resonators have been proposed.
For example, Japanese Unexamined Patent Application Publication No. 2000-315936 discloses a surface acoustic wave branching filter, as shown in FIG. 12. Here, a first surface acoustic wave filter F1 for relatively low frequencies and a second surface acoustic wave filter F2 for relatively high frequencies are connected to an antenna-side common terminal T0. The first surface acoustic wave filter F1, includes a series-arm resonator RS0 and a parallel-arm resonator RP, and the second surface acoustic wave filter F2 includes a parallel-arm resonator RP0 and a series-arm resonator RS.
In the first surface acoustic wave filter F1, a resonator that is closest to the common antenna terminal T0 is the series-arm resonator RS0 and, in the second surface acoustic wave filter F2, a resonator that is closest to the common antenna terminal T0 is the parallel-arm resonator RP0.
Furthermore, a phase rotation line S is sandwiched between the second surface acoustic wave filter F2 and the common antenna terminal T0.
Japanese Unexamined Patent Application Publication No. 10-93382 discloses a surface acoustic wave filter of a ladder-type circuit structure, as shown in FIG. 13. Here, series-arm resonators RS1 and RS2 are connected in series in a series arm between an input terminal and an output terminal. Furthermore, a parallel-arm resonator RP1 is disposed in a parallel arm connected between the input terminal and the series-arm resonator RS1. A parallel-arm resonator RP2 is disposed in a parallel arm, one end of which is connected between the series-arm resonators RS1 and RS2. Moreover, a parallel-arm resonator RP3 is disposed in a parallel arm between the series-arm resonator RS2 and the output terminal.
In this surface acoustic wave filter, the three parallel-arm resonators RP1 to RP3 are commonly connected to a common terminal 51 on a surface acoustic wave chip. Then, the common terminal 51 and the ground terminal of a package are connected by a bonding wire having an inductance LE.
Japanese Unexamined Patent Application Publication No. 5-183380 describes a surface acoustic wave filter of a ladder-type circuit structure wherein the best capacitance ratio is ½ between a parallel-arm resonator disposed at an end portion and a parallel-arm resonator connected in a parallel arm sandwiched between series-arm resonators.
Moreover, Japanese Unexamined Patent Application Publication No. 2001-298348 discloses a surface acoustic wave branching filter, as shown in FIG. 14. As shown in FIG. 14, a first surface acoustic wave filter 61 for relatively low frequencies and a second surface acoustic wave filter 62 for relatively high frequencies are connected to a common terminal 71 on the antenna side in a surface acoustic wave branching filter 70. The surface acoustic wave filters 61 and 62 are surface acoustic wave filters of a ladder-type circuit structure having series-arm resonators S1 to S3 and parallel-arm resonators P1 to P6, respectively.
In a surface acoustic wave branching filter described in Japanese Unexamined Patent Application Publication No. 2000-315936, the series-arm resonator RS0 is connected to a first stage of the first surface acoustic wave filter F1 for relatively low frequencies, and the above-described phase rotation line S is connected to the second surface acoustic wave filter F2.
In recent years, the reduction in size has been strongly required in surface acoustic wave branching filters. Accordingly, when the phase rotation line S is provided in a package, it has been difficult to ensure a sufficient line length for fully rotating the phase. Furthermore, the longer the line length of the phase rotation line S, the larger the resistance of the line. Accordingly, there has been a problem in that the loss of the surface acoustic wave branching filters increases.
On the other hand, when the line length of the phase rotation line S is reduced, the amount of phase rotation becomes small, the impedance matching of the surface acoustic wave filter F2 deviates from a reference impedance of 50 Ω, the loss in the band increases, and there was a concern that isolation characteristics might deteriorate.
In the surface acoustic wave filter described in the above-described Japanese Unexamined Patent Application Publication No. 10-93382, it is stated that the amount of attenuation can be improved by commonly connecting the ground-side terminals of the parallel-arm resonators RP1 to RP3. However, only a technique for improving the amount of attenuation of a surface acoustic wave filter is disclosed, and nothing about the specific structure of series-arm resonators and parallel-arm resonators in a surface acoustic wave branching filter is described.
Furthermore, in Japanese Unexamined Patent Application Publication No. 5-183380, in the surface acoustic wave filter of a ladder-type circuit structure, although the capacitance ratio between a parallel-arm resonator disposed at an end portion and a parallel-arm resonator in a parallel arm disposed between series-arm resonators is described, it is only described to make the capacitance ratio of the parallel-arm resonators a fixed value. That is, nothing is described concerning a desirable structure of the series-arm resonators and the parallel-arm resonators in surface acoustic wave branching filters using a plurality of surface acoustic wave filters.
In the surface acoustic wave branching filter described in the Japanese Unexamined Patent Application Publication No. 2001-298348, the surface acoustic wave branching filter 70 using the surface acoustic wave filters 61 and 62 has the parallel-arm resonator S1 that is closest to the common terminal 71. However, in the surface acoustic wave branching filter 70, a desirable structure of each of the series-arm resonators S1 to S3 and parallel-arm resonators P1 to P6, phase delay circuits, etc., are not specifically mentioned.